Electric distance meter



July 23, 1946. G. PoTAPENKo l 2,404,527

ELECTRIC DISTANCE METER Filed lay 2, 1939 Genera/ar f/5 T if l t s [r I all?! c, INVENTOR,

e/QQ @7b/cent@ ATTORNEY.

Patented July 23, 1946 UNITEDy STATES PATENT OFFICE V.2,404,527 n Y i l Y ELECTRICv DISTANCE METER Gennady Potapenko, Pasadena, Calif.

Application May 2, 1939, Serial No.271,300

7 Claims.

My invention relates to an electric distance meter for measuring the distance between the measuring apparatus and a surface capable ofreiiecting electromagnetic radiation of suitable frequency. The invention has a particularly valuable application to the problem of determiningwhile in an aircraft, the height of the aircraft above the surface of the earth.

An object of the invention is to provide a method and an apparatus for measuring the distance between the measuring apparatus and a surface capable of reecting electromagnetic radiation of suitable frequency.

Another object is to provide a method andan apparatus for measuring elevation above the surface of the earth.

Another object is to provide an apparatus which measures an electrical quantity in the time interval between the sending of a pulse of electromagnetic radiation and its return from a reflecting surface.

Another object of the invention is to provide a method and apparatus for measuring the distance to a surface, in which an electrical measuring instrument is started by an electric pulse coinciding with' a transmitted pulse of radio waves and is stopped by an electric pulse coinciding with a reected pulse of radio waves.

Another object is to provide a method and apparatus for measuring one of two electric pulses only during that portion thereof when the other of said pulses is of substantially Zero amplitude.

A further object is to provide a method and apparatus for producing and measuring an electric current proportional to the time interval between arrival of two electric pulses and independent of the maximum amplitude of the pulses.

Still another object of the invention is to provide an electric distance meter which is accurate and dependable and requires a minimum of attention to operate it.

These and other apparent objects I attain in a manner which will be clear from a consideration of the following description taken in connection with the accompanying drawing, in which:

Fig. l is a diagrammatic illustration of one form of my electric distance meter.

Fig. 2 illustrates the relations existing between two electric pulses existing in the circuit of Fig. 1 for one particular condition.

Fig. 3 illustrates the resultant of the pulses of Fig. 2 and that portion thereof which is measured.

Fig. 4 shows an alternative form of a portion of the circuit of Fig. l.

In the practice of my invention, pulses of elec- (Cl. Z50-1) tromagnetic radiation are transmitted tothe sur' face, the distance of which-is to'be determin'ed5l and the pulses of electromagnetic radiation are received after having beenI reflected from the surface. Agmeasurement of anelectriccurren-tkyrv potential difference derived from the sourcefo'f transmitted radiation, or from a separate source,`

is preferably made inthe interval between trans` mittedand received pulses by initiating themeasurement by one pulse and counteractingwiththel second pulse the action of the first pulse. Since electromagnetic radiation travels with'the substantially constant velocity of approximatelyA 3 10 meters per second'7 the durationof the measurement is an accurate measure of thedis-- tance travelled by the electromagnetic radiation,v

which is substantially twice-the distance fromthe source of radiation tothe reflecting surface.

The method and apparatus of my invention should be distinguished from those previously employed for the measurement of distance `by means of radio waves. Other methods have either measured the elapsed time: between .transmission and reception of the waves directly by means of a cathode beam tube or havemodulated the transmitter frequency linearly and have determined by a beat method the. difference-"infrequency between waves being receivedand those being transmitted. 'Ihe latter method is de'- scribed in an article by S. Matsuo, Proc. I. R.. E., vol. 26, p. 848. The apparatus of these methods is bulky and requires considerable attention on the part of an operator. The apparatus Vof my invention, on the other hand, is relativelylight, vis easy to operate, and requires no attention other than to read `the numerical registration of Adise I provide a short wave radio transmitter lll, preferably adapted to rgenerate alternating current of a frequency between 10 and 109 cycles per second. The transmitter is keyed by means of aV keying device generally designated by thenu meral ll to produce a series of pulses of high frequency alternating potential diierence be tween the conductors I2 land-I3. For theV sake of diagrammatic illustration, Vthe keying device It vals.

difference which appears at .the beginning of a pulse, continues at substantially constant amplitude during the pulse, and disappearsfat the end of the pulse. Preferably the frequency of the pulses is from 103 to 104 per second, although this depends upon the desired range of the distance meter, as hereinafter explained. It will be seen, then, that within a single pulse, as it exists" at conductors I2 and I3, there are many radio-frequency cycles, and since during a pulse the amplitude of the radio-frequency variation is preferably substantially vthe same in each cycle, a plot of the amplitude of the radio-frequency potential difference against time would be rectangular in shape and appear similar to the rectangles d in Fig. 2. While pulses of rectangular shape are shown, it will be understood that pulses of-Various other suitable shapes may be employed, if desired.

It will be understood that the keying device is shown as illustrated in Fig. l purely for the sake of simplicity. A switch in the conductor I2 or within the transmitter I0 may, if desired, be operated by a mechanical device adapted to open and close it at predetermined inter- Or any one of the keying devices wellknown in the art may be employed to open and close the circuit from the transmitter or to provide o and o periods of the current from the transmitter at predetermined intervals. The transmitter may, if desired, be modulated in a well-known manner by means of electronic devices to eiect this result.

A directional transmitting antenna I6 is connected by means of conductors I1 and I8 to the conductors I2 and I3, respectively, the antenna being connected with the conductors in a manner well-known in the art to provide efficient directional transmission of short-wave radio waves therefrom. While various arrangements known in the art may be employed to provide directional transmission, a parabolic reflector 20 is shown herein for that purpose.

Conductors 2| and 22 form continuations of the conductors I2 and I3, respectively. In series with the conductor 2| is placed a rectier 23 which may, for example, permit current to pass along the conductor 2|,away from the conductor I2. Instead of the half-wave rectier s hown, I may, if desired, employed a full-wave rectier. A lter 25 is connected between the incoming conductors 2| and 22 and outgoing conductors 26 and 21, and is adapted to filter out substantially all of the radio-frequency component of the rectified current through conductors 2| and 22, leaving in conductors 26 and 21 substantially only pulses of continuous direct current, of vsubstantially rectangular wave shape, designated by d in Fig. 2. The filter 25 is preferably of the concentric line type such, for instance, as that described in an article lby Leeds, Proceedings of the Institute of Radio Engineers, vol. 26. pp. 576-589. A :filter of this type comprises a cylinder 28 concentric with the incoming line 2| and 4 preferably of a length equal to one-half wave length of the radio-frequency current desired to be ltered out. The incoming conductor 22 and the outgoing conductor 21 are both connected to the cylinder 28 and the outgoing conductor 26 is connected to the conductor 2| at a point midway between the ends of the cylinders 28 and passes out through a hole in the cylinder. Cylinders of otherl lengths may, if desired, be employed and suitable connectionsbe'made thereto to eifect the same result, all as described in the above-mentioned article. Filters of this type are "considered more desirable for very high frequencies than filters With lumped capacity and inductance. In the embodiment illustrated in Fig. 1,afresistance 33 is connected between the conductors 26 and 21.

A directional receiving antenna 3|, which may be provided with a parabolic reector 32 to give it directional properties, is connected to a receiver and radio-frequency amplifier 33 through conductors 34 and 35. Conductors 36 and 31.are connected to the outgoing side of the receiver 33. In series with the conductor 31 is placed the rectifier 38 which may be of either the half-wave or full-Wave type and is adapted to pass current away from the receiver 33; and between the conductors 36 and 31, beyond vthe rectifier, is preferably connected a lter 4D, preferably of the concentric line type, which is similar to the filter 25 and is adapted to lter out the radio-frequency component of the current in the conductors 36 and 31, leaving in the outgoing conductors 4| and 42 substantially only pulses of continuous direct current ofV substantially rectangular wave shape, designated by the lettervr in Fig. 2. In the embodiment of Fig. l, a resistance 43 is connected between the conductors 4| and 42 and the conductors 21 and 4| are connected together.

In the embodiment of Fig. 1, conductors 44 and 45 are connected, respectively, to the conductors 26 and 42. A rectier 46 is connected in series with the conductor 44 and is adapted to pass current away from the conductor 26. An electric measuring instrument 41 preferably of relatively high resistance compared to the resistances 30 and 43, and preferably of ballistic character, is connected between the conductors 44 and 45 beyond the rectifier 46, and is adapted to measure the average current in the conductors 44 and 45 or the average potential difference therebetween. When the conductor 42 is positive with respect to the conductor 26 current is prevented by the rectifier 46 from passing through the instrument 41, and it is only when the ,conductor 26 is more positive than the conductor 42 that the instrument 41 is operative. It is understood that variousbridge circuits and other connections of the electrical measuring instrument may be made to accomplish similar results.

As employed on an airplaneto measure the elevation of the airplane above thev surface of the earth, the antenna I6 is preferably located under one wing with the reilector 2U positioned so as to direct the radiation downwardly. The antenna 3| is preferably located under the other wing on the opposite side of the fuselage from the antenna I 6 so as to minimize direct reception by the antenna 3| of the radiation from the antenna I6, and the reilector 32 is so positioned as to cause the antenna 3| to receive radiation predominantly coming from below. It will be understood, however, that any other `suitable arrangement of the antenna may be employed.

In operation, the conductors I1, .IB .and 2I, 22 are subjected to pulses of high frequency electromotive force. At the time t1, in Fig. 2., a pulse may begin and may continue until a later time t3. During this interval .a high frequency electromotve force is impressed upon the conductors I1 and I8 and also upon the conductors 2| and 22. During this interval, therefore, high frequency radio waves are radiated from the antenna I6 toward the earth. In the interval between the times t3 and te no potential diierence exists between the conductors I1 and I 8 or between the conductors 2| and 22. Between the times t5 and tv another pulse .of radio-frequency electromotive force is impressed .upon the conductors I1, I8 and upon the conductors 2i, 22.

During the pulses between times t1 to ts and t5 to t1 the radio-frequency potential difference between the conductors 2i and `22 is rectified by the rectifier 23 and varying direct current tends to flow through the filter 25 and the resistance 30. The filter 25 removes substantially all of the high frequency component of the current in the circuit, leaving substantially a continuous direct current which varies similarly to the rectangle d in Fig. 2.

The radio-frequency waves radiated from the antenna I6 pass downwardly to the earths surface from which they are reflected upwardly and are received on the antenna 3| at a time following their radiation from the antenna I5 by an interval depending upon the distance traveled by the waves. The airplane makes very little progress in the short interval between radiation and reception of the waves and the elapsed time in seconds is substantially equal to two times the elevation in meters of the airplane above the surface of the earth divided by BXlOS, the velocity of light in meters per second. The radio-frequency waves impinging upon the antenna 3l are translated by the receiver and amplier 33 into a radio-frequency potential difference across the conductors 33 and -31 which is rectified by the rectifier 38 and tends to produce a varying direct current through the ilter 40 and the resistance 43. The lter 46 removes substantially all of the radio-frequency component and leaves a continuous direct current of amplitude Ir through the resistance 43 which varies similarly to the rectangle r in Fig. 2. Since the effect of the current Ir in the measuring circuit is opposed to that of the current Id, the current Ir is, for illustrative purposes, shown on the opposite -side of the O line in the chart of Fig. 2. The amplification of the amplifier 33 is preferably adjusted so that the potential diiierence across the resistance 43 due to the passage of the current Ir therethrough is greater than the potential difference across the resistance 30 due to the passage of the current Id therethrough. It will be understood that it is not necessary that the resistances 30 and 43 be of the same value but it is assumed here that this condition exists so that the potential differences across the respective resistances are proportional to the currents flowing therethrough.

For a particular elevation of the airplane above the earths surface, the interval oi time elapsing between the radiation and reception of a pulse may be tltz. The pulse d of current Id through resistance 30 will` then, start at the time tiwh-ile the pulse T of current Ir through the resistance 43 will not start until the time t2, and whereas the pulse d will end at t3 the pulse r will not end until t4, the interval t1t2 being equal to the interval tati. Likewise, the next pulse r will start at 6 time Lts 4and end at time At8 whereithe intervals ists and tata are both equal to thelintervals titz and 3154. Y* Y During the `time interval t-itza Apotential difference .exists across lthe resistance 30 proportional to the-current Id and substantially no potential difference exists .across the resistance 43. The

conductor is therefore ypositive with respect to the conductor 42 and a potential diierence exists across the instrument 41 proportional to the current Id. 'In the interval tzta the current Ir through the resistance 43 counteracts the effect of the current Id through the resistance 3D and there eX- ists a potential difference between theconductors 42 and-*243 yproportional to the diiierence -Is vbetween Ir and Id,` with the conductor 42 positive with respect to the conductor 26. Since, under these conditions, the rectifier 46 does not permit any current to pass, there is `no potential diier ence across the instrument 41 during the interval tats. During the interval 'tati no potential difference exists across the resistance 30 anda potential difference exists across the resistance 43 proportional to the current Ir, the conductor-42 being positive with respect to the conductor '26. No potential diierence, therefore, exists across the instrument `41 during the period titi. During the period tits no current flows through either ofthe resistances 30 and 43 and nopotential dierence exists vacross the instrument 41, Upon the vstart of another pulse the instrument again .registers a potential difference proportional to Id during the interval ists and is idle 'until the succeeding pulse d occurs.

In Fig, 3 isplctted the potential diierence between the lconductors 26 and 42, with those intervals during which measurement takes place being cross-hatched. For a given frequency of pulses it Vwill be evident that, since the value. of Id is constant, the reading of the instrument 41 will be proportional to IdXAt where At equals the time between `t1 and t2. As previously explained, the time intervalnt required for the radiation to travel to the earthssurface and 'back is proportional to the elevation H of the airplane above the surface of the earth. It will be evident then that the reading vof the instrument 41 will be proportional to IdXH. The instrument 41 may, therefore, lbe calibrated directly in terms of H.

Itis important that the durationltits of each pulse be at least as `long as the interval titz corresponding to the greatest distance desired to be measured, -and that the interval tats be at least as long as the interval tits. Preferably the in-V requires :a separate calibration for each pulse frequency employed.

Due to vdirect coupling between thetransmitting and receiving antennas vit may be that a small current will flow through the resistance 43 coincident-ally with the passage of the current ld through the resistance 130, but since it will start and stop at the same times as the current Id it will beevident that the only eiect will beto counteract some of the current Id. Since the amplitude of this current due to direct coupling' is constant, its eiectupon the reading ofthe inn strument 41 will be taken account of in calibration of' the instrument.

An alternative form of measuring circuit is illustrated in Fig. 4. Two vacuum tubes 50 and 5|, each with two grids, are preferably employed. Heaters 52 and 53 of the tubes 5U and 5|, respectively, are connected in parallel and are supplied by a battery 54. The cathodes 55 and 56 of the tubes 50 and 5|, respectively, are connected together and to the negative side of a battery 51 and to the conductor 21 and 42. The anode 58 of the tube 50 is connected to the positive side cf the battery 51 through a resistance 60, and the anode'6l of the tube 5| is preferably connected to the positive side of the battery 51 through a resistance 62 and a current measuring instrument 63. One of the grids 64 of the tube 56 is connected to a point between the resistance 62 and the meter 63 while the other of the grids 65 is connected to the conductor 26, One of the grids 66 of the tube 5| is connected to the anode 58 and the other of the grids 61 is connected to the conductor 4|. A resistance 68 is preferably connected between the conductors 26 and 21 and another resistance 10 is preferably connected between the conductors 4| and 42.

The Passage of plate current I1 through the resistance 60 places a negative bias on the grid 66 of tube and the passage of plate current I2 through the resistance 62 places a negative bias on the grid 64 of the tube 50. The resistances 60 and 62 and the voltage of the battery 51 are made such that the biases resulting from passage of the currents I1 and Iz are beyond the cut-01T points of the tubes 5| and 50. The resistances 68 and 10\are of such Values th'at when the currents Id and Ir, respectively, pass through them, the negative biases produced on the grids 65 and 61 are suiiicient to reduce the currents in the respective tubes to substantially zero.

The operation of the circuit is, then, as follows. Assume that in the interval prior to t1 the tube 50 is conducting and plate current I1 is flowing. The grid 66 is biased beyond cut-oir and no current ows through the meter 63. At time t1 the current Id starts to flow through the resistance 68, the grid 65 is biased beyond cut-oi, and the current I1 ceases to flow. The bias on grid 66 then no longer exists and the tube 5| immediately becomes conducting, the current I2 passing through the meter 63 and the resistance 62. The passage of the current Iz still further biases the grid 64 of the tube 56. The current I2 continues to ow until time t2 when the passage of current Ir through the resistance biases the grid 61 to such an extent that the current I2 is stopped. During the interval tzta both tubes are non-conducting. At the time t3 the bias on the grid 65 is removed due to stopping of the current Id and the current I1 starts flowing, biasing the grid 66. When the bias on the grid 61 is removed at t4 due to stopping of the current Ir the bias on the grid 66 still prevents current from flowing through the meter 63 and the tube 5|. The current I1 continues to flow through the tube 50 until another pulse starts at t5 and the entire cycle is repeated. It will be seen that current flows through the meter 63 only during the time intervals titz, tsts, etc. Since the current which i-lows is constant while it flows and is independent of the intensity of the radiation received on the antenna 3|, the reading of the meter 63 will vary only with the length of the time interval At, which is proportional to the height of the apparatus above the surface of the earth.

The circuit of Fig. 4 has only ohmic resistances and inter-electrode capacities which Yare small. The time constants of various parts of the circuit are therefore Very small, and by proper choice of tubes, resistances, and batteries it is possible to make the circuit operate at extremely high speed so that the time interval titz can still be recognized even though it is only of the order of a small fraction of 1 microsecond. When the circuit of Fig. 4 is 'employed the filters 25 and 40 may be eliminated, if desired. While I have, in Fig. 4, shown a particular circuit and tubes with two grids it will be understood that the same principles may be applied to provide other vacuum tube relay circuits employing tubes as described or tubes of other well-known types.

When a vacuum tube relay circuit such as that of Fig. 4 is employed the pulse duration tits may be made as short as desired so long as it is long enough to operate the relay. With this lcircuit it is not necessary that the interval tits be as long as t1t'2. This will be evident from the fact that current I2 starts to now at time t1 and will, by reason of the resulting bias on the grid 64, continue to flow even if current Id stops owing and the bias on grid 65 is removed. The current I2 will continue to flow until pulse r arrives at the time t2. It is still necessary, however, for the interval tate between pulses to be equal to or greater than the interval At corresponding to the greatest distance desired to be measured. One possible arrangement is to make the intervals tits, tet?, etc., always substantially equal to the interval At. This result may be automatically secured by arranging the transmitter keying relay to be operated by the pulse generator to key the transmitter on and to be energized by the potential difference between the conductors 4| and 42 to key the transmitter o when the pulse 1' arrives at t2. But, in practice, it is preferable to operate the apparatus, as previously described, wherein the intervals tits, tstv, etc., are independent of At and the effect 0f the pulse coinciding with the transmitted pulse is counteracted by the pulse coinciding with the pulse received after reflection, the counteraction being eiTected in a circuit totally separated from the keying relay, as shown in Figs. 1 and 4. Under such circumstances, the keying of the transmitter and the load on it are independent of the operation of the rest of the apparatus and the stability of the transmitter is more easily maintained.

It will be seen that the apparatus disclosed is adapted to function within wide limits of the intensity of the radiation received after having been reflected from the distant surface, for the operation of my apparatus depends not upon the intensity of the received radiation but upon its time of arrival relative to the time when it was sent. Another great advantage of my apparatus is that a definite numerical registration of distance is continuously given without the necessity of any setting or adjustment operation to obtain the measurement.

It will be understood that various changes and modications in the apparatus disclosed may be made by those skilled in the art without departing from the spirit and scope of the invention as dened in the appended claims.

I claim as my invention:

1. Distance measuring apparatus, comprising: an antenna adapted to radiate toward a surface; means including a radio transmitter adapted to periodically energize said antenna, whereby pulses of radio waves are radiated to.-

9a; wardrsaid surface; an electric circuit energized by said transmitter coincidentally withsaid antenna; a. rectifier in said circuit,v and a filter in said circuit for ltering out radi'o-frequencycomponents of the current in said circuit, whereby pulses of continuous direct current corresponding substantially intime with said transmitted.

pulses pass. through said electric circuit; means including an antenna. and a radio-frequency ampliner for receiving said pulses of radio waves after having been reflected from said surface; a second electric circuit connected to said receiving means; a rectifier in said second circuit, and a filter in said second circuit for ltering out radio-frequency components of the current in said second circuit, whereby pulses of continuous direct current corresponding substantially in time with said received pulses pass through said second circuit; an electrical measuring instrument; and an electric circuit connected with said instrument and with said iirst and second circuits and adapted to energize said instrument substantially only in response to a pulse passing in said first circuit corresponding substantially in time with one of said transmitted pulses and in the absence of any pulse passing in said second circuit corresponding in time with a received pulse.

2. Distance measuring apparatus, comprising: an antenna adapted to radiate toward a surface; means including a radio transmitter adapted to periodically energize said antenna, whereby pulses of radio waves are radiated toward said surface; an electric circuit energized by said transmitter coincidentally with said antenna; a rectifier in said electric circuit, and a filter in said electric circuit for iiltering out radiofrequency components of the current in said circuit, whereby pulses of continuous direct current corresponding substantially in time with said transmitted pulses pass through said electric circuit; means including an antenna and a radiofrequency amplier for receiving said pulses of radio waves after having been reliected from said surface; a second electric circuit connected to said receiving means; a rectifier in said second electric circuit, and a lter in said second electric circuit for iiltering out radio-frequency components of the current in said circuit, whereby pulses of continuous direct current corresponding substantially in time with said received pulses pass through said second circuit; and an electric circuit connected to said rst and second circuits and including electrical measuring means responsive to said pulses of direct current in one of said circuits during the periods when said pulses of direct current in the other of said circuits are not passing therethrough.

3. Distance measuring apparatus, comprising: an antenna adapted to radiate toward a surface; means including a radio transmitter adapted to periodically energize said antenna, whereby pulses of radio waves are radiated toward said surface; an electric circuit energized 'by said transmitter coincidentally with said antenna; a rectier in said electric circuit, and a filter in said electric circuit for filtering out radiofrequency components of the current in said circuit, whereby pulses of continuous direct current corresponding substantially in time with said transmitted pulses pass through said electric circuit; means including an antenna and a radiofrequency amplifier for receiving said pulses of radio waves after having been reflected from said Surface; a second electric circuit connected to 1,0`- said receiving means.; a rectifier in said second electric circuit, anda lter in said second electric circuit for filtering out radio-frequency coniponentsof the current in- 'said circuit, whereby pulsesA of continuous directl currentl corresponding substantiallyin time with said received pulses pass through said second circuit, said receiving` means being sov adjusted that said direct currentpulses in said second circuit are or greater amplitude than said direct current pulses in' said-first first and second circuits andr including measuring means responsive to the current iii-said rst circuit when it is greater than the ,currentr in said second circuit. Y

4:. Distancey measuring apparatus, comprising: an antennaadapted to radiate toward a surface; means including a radio transmitter adapted to periodically energize said antenna, whereby pulses of radio waves are radiated toward said surface; an electric circuit energized by said transmitter coincidentally with said antenna; a rectifier in said electric circuit, whereby pulses of direct current corresponding substantially in time with said transmitted pulses pass through said circuit; means including an antenna and a radio-frequency amplifier for receiving said pulses of radio waves after having been reflected from said surface; a second electric circuit connected to said receiving means; a rectifier in said second electric circuit, whereby pulses of direct current corresponding substantially in time with said received pulses pass through said second circuit; a lter in eachA of said circuits for iiltering out the radio-frequency components of the currents in said circuits; and an electric i circuit connected to said rst and second circuits and including measuring means responsive means including a radio transmitter adapted to whereby periodically energize said antenna, pulses of radio waves are radiated toward said surface; means including an antenna and a radio-frequency amplifier for receiving said pulses of radio waves after having been reflected from said surface; an electric circuit energized by said transmitter coincidentally with said antenna; a second electric circuit connected to said receiving means; a rectiiier in each of said circuits, whereby pulses of direct current corresponding in time substantially with said transmitted and said reiiected pulses pass respectively through said iirst and said second circuits; an electrical measuring instrument; and means in connection with said instrument and with said first and second circuits for starting a `predetermined current through said instrument at the Vstart of each pulse in said first circuit v means for producing continuous direct current pulses in said circuit substantially coinciding in time with said transmitted pulses of electromagnetic radiation; means for receiving said pulses of electromagnetic radiation after having 'been reflected from said surface; means including a second electric circuit for translating saidv received pulses intocontinuous direct current pulses in said second circuit substantially coinciding in time With said received pulses; an electrical measuring instrument; and means including an electric circuit connected with said instrument and with said first and second circuits and responsive to the direct current pulses in said first and second circuits for energizing said instrument only during time intervals determined by the time lag between the direct current pulses in said rst circuit and the direct current pulses in said second circuit.

'7. Distance measuring apparatus, comprising: means for transmitting pulses of electromagnetic radiation to a surface; means including an electric circuit associated With said transmitting means for producing continuous direct current pulses in Said circuit substantially, coinciding in time with said transmitted pulses of electromagnetic radiation; means for receiving said pulses of electromagnetic radiation afterliaving been reected from said surface; .means including a second electric circuit for translating said, received pulses into continuous direct current pulses in said second circuit substantially coinciding in time .with said received pulses;V4 and means, including an electrical measuring instrument and an electric 'circuit connected with said instrument and with said first and second cir--v cuits, for measuring the time lag between said direct current pulses in said first circuit and said direct current pulses in said second circuit.

GENNADY POTAPENKO. 

